// Copyright (c) 2012 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

// Time represents an absolute point in coordinated universal time (UTC),
// internally represented as microseconds (s/1,000,000) since the Windows epoch
// (1601-01-01 00:00:00 UTC). System-dependent clock interface routines are
// defined in time_PLATFORM.cc. Note that values for Time may skew and jump
// around as the operating system makes adjustments to synchronize (e.g., with
// NTP servers). Thus, client code that uses the Time class must account for
// this.
//
// TimeDelta represents a duration of time, internally represented in
// microseconds.
//
// TimeTicks and ThreadTicks represent an abstract time that is most of the time
// incrementing, for use in measuring time durations. Internally, they are
// represented in microseconds. They can not be converted to a human-readable
// time, but are guaranteed not to decrease (unlike the Time class). Note that
// TimeTicks may "stand still" (e.g., if the computer is suspended), and
// ThreadTicks will "stand still" whenever the thread has been de-scheduled by
// the operating system.
//
// All time classes are copyable, assignable, and occupy 64-bits per
// instance. Thus, they can be efficiently passed by-value (as opposed to
// by-reference).
//
// Definitions of operator<< are provided to make these types work with
// DCHECK_EQ() and other log macros. For human-readable formatting, see
// "base/i18n/time_formatting.h".
//
// So many choices!  Which time class should you use?  Examples:
//
//   Time:        Interpreting the wall-clock time provided by a remote
//                system. Detecting whether cached resources have
//                expired. Providing the user with a display of the current date
//                and time. Determining the amount of time between events across
//                re-boots of the machine.
//
//   TimeTicks:   Tracking the amount of time a task runs. Executing delayed
//                tasks at the right time. Computing presentation timestamps.
//                Synchronizing audio and video using TimeTicks as a common
//                reference clock (lip-sync). Measuring network round-trip
//                latency.
//
//   ThreadTicks: Benchmarking how long the current thread has been doing actual
//                work.

#ifndef BASE_TIME_TIME_H_
#define BASE_TIME_TIME_H_

#include <stdint.h>
#include <time.h>

#include <iosfwd>
#include <limits>

#include "base/base_export.h"
#include "base/compiler_specific.h"
#include "base/numerics/safe_math.h"
#include "build/build_config.h"

#if defined(OS_MACOSX)
#include <CoreFoundation/CoreFoundation.h>
// Avoid Mac system header macro leak.
#undef TYPE_BOOL
#endif

#if defined(OS_POSIX)
#include <sys/time.h>
#include <unistd.h>
#endif

#if defined(OS_WIN)
// For FILETIME in FromFileTime, until it moves to a new converter class.
// See TODO(iyengar) below.
#include "base/gtest_prod_util.h"
#include <windows.h>
#endif

#undef max
#undef min

namespace base {

class PlatformThreadHandle;
class TimeDelta;

// The functions in the time_internal namespace are meant to be used only by the
// time classes and functions.  Please use the math operators defined in the
// time classes instead.
namespace time_internal {

    // Add or subtract |value| from a TimeDelta. The int64_t argument and return
    // value are in terms of a microsecond timebase.
    BASE_EXPORT int64_t SaturatedAdd(TimeDelta delta, int64_t value);
    BASE_EXPORT int64_t SaturatedSub(TimeDelta delta, int64_t value);

    // Clamp |value| on overflow and underflow conditions. The int64_t argument and
    // return value are in terms of a microsecond timebase.
    BASE_EXPORT int64_t FromCheckedNumeric(const CheckedNumeric<int64_t> value);

} // namespace time_internal

// TimeDelta ------------------------------------------------------------------

class BASE_EXPORT TimeDelta {
public:
    TimeDelta()
        : delta_(0)
    {
    }

    // Converts units of time to TimeDeltas.
    static /*constexpr*/ TimeDelta FromDays(int days);
    static /*constexpr*/ TimeDelta FromHours(int hours);
    static /*constexpr*/ TimeDelta FromMinutes(int minutes);
    static /*constexpr*/ TimeDelta FromSeconds(int64_t secs);
    static /*constexpr*/ TimeDelta FromMilliseconds(int64_t ms);
    static /*constexpr*/ TimeDelta FromSecondsD(double secs);
    static /*constexpr*/ TimeDelta FromMillisecondsD(double ms);
    static /*constexpr*/ TimeDelta FromMicroseconds(int64_t us);
#if defined(OS_WIN)
    static TimeDelta FromQPCValue(LONGLONG qpc_value);
#endif

    // Converts an integer value representing TimeDelta to a class. This is used
    // when deserializing a |TimeDelta| structure, using a value known to be
    // compatible. It is not provided as a constructor because the integer type
    // may be unclear from the perspective of a caller.
    static TimeDelta FromInternalValue(int64_t delta) { return TimeDelta(delta); }

    // Returns the maximum time delta, which should be greater than any reasonable
    // time delta we might compare it to. Adding or subtracting the maximum time
    // delta to a time or another time delta has an undefined result.
    static TimeDelta Max();

    // Returns the internal numeric value of the TimeDelta object. Please don't
    // use this and do arithmetic on it, as it is more error prone than using the
    // provided operators.
    // For serializing, use FromInternalValue to reconstitute.
    int64_t ToInternalValue() const { return delta_; }

    // Returns the magnitude (absolute value) of this TimeDelta.
    TimeDelta magnitude() const
    {
        // Some toolchains provide an incomplete C++11 implementation and lack an
        // int64_t overload for std::abs().  The following is a simple branchless
        // implementation:
        const int64_t mask = delta_ >> (sizeof(delta_) * 8 - 1);
        return TimeDelta((delta_ + mask) ^ mask);
    }

    // Returns true if the time delta is zero.
    bool is_zero() const
    {
        return delta_ == 0;
    }

    // Returns true if the time delta is the maximum time delta.
    bool is_max() const { return delta_ == std::numeric_limits<int64_t>::max(); }

#if defined(OS_POSIX)
    struct timespec ToTimeSpec() const;
#endif

    // Returns the time delta in some unit. The F versions return a floating
    // point value, the "regular" versions return a rounded-down value.
    //
    // InMillisecondsRoundedUp() instead returns an integer that is rounded up
    // to the next full millisecond.
    int InDays() const;
    int InHours() const;
    int InMinutes() const;
    double InSecondsF() const;
    int64_t InSeconds() const;
    double InMillisecondsF() const;
    int64_t InMilliseconds() const;
    int64_t InMillisecondsRoundedUp() const;
    int64_t InMicroseconds() const;

    TimeDelta& operator=(TimeDelta other)
    {
        delta_ = other.delta_;
        return *this;
    }

    // Computations with other deltas.
    TimeDelta operator+(TimeDelta other) const
    {
        return TimeDelta(time_internal::SaturatedAdd(*this, other.delta_));
    }
    TimeDelta operator-(TimeDelta other) const
    {
        return TimeDelta(time_internal::SaturatedSub(*this, other.delta_));
    }

    TimeDelta& operator+=(TimeDelta other)
    {
        return *this = (*this + other);
    }
    TimeDelta& operator-=(TimeDelta other)
    {
        return *this = (*this - other);
    }
    TimeDelta operator-() const
    {
        return TimeDelta(-delta_);
    }

    // Computations with numeric types.
    template <typename T>
    TimeDelta operator*(T a) const
    {
        CheckedNumeric<int64_t> rv(delta_);
        rv *= a;
        return TimeDelta(time_internal::FromCheckedNumeric(rv));
    }
    template <typename T>
    TimeDelta operator/(T a) const
    {
        CheckedNumeric<int64_t> rv(delta_);
        rv /= a;
        return TimeDelta(time_internal::FromCheckedNumeric(rv));
    }
    template <typename T>
    TimeDelta& operator*=(T a)
    {
        return *this = (*this * a);
    }
    template <typename T>
    TimeDelta& operator/=(T a)
    {
        return *this = (*this / a);
    }

    int64_t operator/(TimeDelta a) const { return delta_ / a.delta_; }
    TimeDelta operator%(TimeDelta a) const
    {
        return TimeDelta(delta_ % a.delta_);
    }

    // Comparison operators.
    /*constexpr*/ bool operator==(TimeDelta other) const
    {
        return delta_ == other.delta_;
    }
    /*constexpr*/ bool operator!=(TimeDelta other) const
    {
        return delta_ != other.delta_;
    }
    /*constexpr*/ bool operator<(TimeDelta other) const
    {
        return delta_ < other.delta_;
    }
    /*constexpr*/ bool operator<=(TimeDelta other) const
    {
        return delta_ <= other.delta_;
    }
    /*constexpr*/ bool operator>(TimeDelta other) const
    {
        return delta_ > other.delta_;
    }
    /*constexpr*/ bool operator>=(TimeDelta other) const
    {
        return delta_ >= other.delta_;
    }

private:
    friend int64_t time_internal::SaturatedAdd(TimeDelta delta, int64_t value);
    friend int64_t time_internal::SaturatedSub(TimeDelta delta, int64_t value);

    // Constructs a delta given the duration in microseconds. This is private
    // to avoid confusion by callers with an integer constructor. Use
    // FromSeconds, FromMilliseconds, etc. instead.
    /*constexpr*/ explicit TimeDelta(int64_t delta_us)
        : delta_(delta_us)
    {
    }

    // Private method to build a delta from a double.
    static /*constexpr*/ TimeDelta FromDouble(double value);

    // Private method to build a delta from the product of a user-provided value
    // and a known-positive value.
    static /*constexpr*/ TimeDelta FromProduct(int64_t value, int64_t positive_value);

    // Delta in microseconds.
    int64_t delta_;
};

template <typename T>
inline TimeDelta operator*(T a, TimeDelta td)
{
    return td * a;
}

// For logging use only.
BASE_EXPORT std::ostream& operator<<(std::ostream& os, TimeDelta time_delta);

// Do not reference the time_internal::TimeBase template class directly.  Please
// use one of the time subclasses instead, and only reference the public
// TimeBase members via those classes.
namespace time_internal {

    // TimeBase--------------------------------------------------------------------

    // Provides value storage and comparison/math operations common to all time
    // classes. Each subclass provides for strong type-checking to ensure
    // semantically meaningful comparison/math of time values from the same clock
    // source or timeline.
    template <class TimeClass>
    class TimeBase {
    public:
        static const int64_t kHoursPerDay = 24;
        static const int64_t kMillisecondsPerSecond = 1000;
        static const int64_t kMillisecondsPerDay = kMillisecondsPerSecond * 60 * 60 * kHoursPerDay;
        static const int64_t kMicrosecondsPerMillisecond = 1000;
        static const int64_t kMicrosecondsPerSecond = kMicrosecondsPerMillisecond * kMillisecondsPerSecond;
        static const int64_t kMicrosecondsPerMinute = kMicrosecondsPerSecond * 60;
        static const int64_t kMicrosecondsPerHour = kMicrosecondsPerMinute * 60;
        static const int64_t kMicrosecondsPerDay = kMicrosecondsPerHour * kHoursPerDay;
        static const int64_t kMicrosecondsPerWeek = kMicrosecondsPerDay * 7;
        static const int64_t kNanosecondsPerMicrosecond = 1000;
        static const int64_t kNanosecondsPerSecond = kNanosecondsPerMicrosecond * kMicrosecondsPerSecond;

        // Returns true if this object has not been initialized.
        //
        // Warning: Be careful when writing code that performs math on time values,
        // since it's possible to produce a valid "zero" result that should not be
        // interpreted as a "null" value.
        bool is_null() const
        {
            return us_ == 0;
        }

        // Returns true if this object represents the maximum time.
        bool is_max() const { return us_ == std::numeric_limits<int64_t>::max(); }

        // Returns the maximum time, which should be greater than any reasonable time
        // with which we might compare it.
        static TimeClass Max()
        {
            return TimeClass(std::numeric_limits<int64_t>::max());
        }

        // For serializing only. Use FromInternalValue() to reconstitute. Please don't
        // use this and do arithmetic on it, as it is more error prone than using the
        // provided operators.
        int64_t ToInternalValue() const { return us_; }

        TimeClass& operator=(TimeClass other)
        {
            us_ = other.us_;
            return *(static_cast<TimeClass*>(this));
        }

        // Compute the difference between two times.
        TimeDelta operator-(TimeClass other) const
        {
            return TimeDelta::FromMicroseconds(us_ - other.us_);
        }

        // Return a new time modified by some delta.
        TimeClass operator+(TimeDelta delta) const
        {
            return TimeClass(time_internal::SaturatedAdd(delta, us_));
        }
        TimeClass operator-(TimeDelta delta) const
        {
            return TimeClass(-time_internal::SaturatedSub(delta, us_));
        }

        // Modify by some time delta.
        TimeClass& operator+=(TimeDelta delta)
        {
            return static_cast<TimeClass&>(*this = (*this + delta));
        }
        TimeClass& operator-=(TimeDelta delta)
        {
            return static_cast<TimeClass&>(*this = (*this - delta));
        }

        // Comparison operators
        bool operator==(TimeClass other) const
        {
            return us_ == other.us_;
        }
        bool operator!=(TimeClass other) const
        {
            return us_ != other.us_;
        }
        bool operator<(TimeClass other) const
        {
            return us_ < other.us_;
        }
        bool operator<=(TimeClass other) const
        {
            return us_ <= other.us_;
        }
        bool operator>(TimeClass other) const
        {
            return us_ > other.us_;
        }
        bool operator>=(TimeClass other) const
        {
            return us_ >= other.us_;
        }

        // Converts an integer value representing TimeClass to a class. This is used
        // when deserializing a |TimeClass| structure, using a value known to be
        // compatible. It is not provided as a constructor because the integer type
        // may be unclear from the perspective of a caller.
        static TimeClass FromInternalValue(int64_t us) { return TimeClass(us); }

    protected:
        explicit TimeBase(int64_t us)
            : us_(us)
        {
        }

        // Time value in a microsecond timebase.
        int64_t us_;
    };

} // namespace time_internal

template <class TimeClass>
inline TimeClass operator+(TimeDelta delta, TimeClass t)
{
    return t + delta;
}

// Time -----------------------------------------------------------------------

// Represents a wall clock time in UTC. Values are not guaranteed to be
// monotonically non-decreasing and are subject to large amounts of skew.
class BASE_EXPORT Time : public time_internal::TimeBase<Time> {
public:
    // The representation of Jan 1, 1970 UTC in microseconds since the
    // platform-dependent epoch.
    static const int64_t kTimeTToMicrosecondsOffset;

#if !defined(OS_WIN)
    // On Mac & Linux, this value is the delta from the Windows epoch of 1601 to
    // the Posix delta of 1970. This is used for migrating between the old
    // 1970-based epochs to the new 1601-based ones. It should be removed from
    // this global header and put in the platform-specific ones when we remove the
    // migration code.
    static const int64_t kWindowsEpochDeltaMicroseconds;
#else
    // To avoid overflow in QPC to Microseconds calculations, since we multiply
    // by kMicrosecondsPerSecond, then the QPC value should not exceed
    // (2^63 - 1) / 1E6. If it exceeds that threshold, we divide then multiply.
    enum : int64_t { kQPCOverflowThreshold = 0x8637BD05AF7 };
#endif

    // Represents an exploded time that can be formatted nicely. This is kind of
    // like the Win32 SYSTEMTIME structure or the Unix "struct tm" with a few
    // additions and changes to prevent errors.
    struct BASE_EXPORT Exploded {
        int year; // Four digit year "2007"
        int month; // 1-based month (values 1 = January, etc.)
        int day_of_week; // 0-based day of week (0 = Sunday, etc.)
        int day_of_month; // 1-based day of month (1-31)
        int hour; // Hour within the current day (0-23)
        int minute; // Minute within the current hour (0-59)
        int second; // Second within the current minute (0-59 plus leap
            //   seconds which may take it up to 60).
        int millisecond; // Milliseconds within the current second (0-999)

        // A cursory test for whether the data members are within their
        // respective ranges. A 'true' return value does not guarantee the
        // Exploded value can be successfully converted to a Time value.
        bool HasValidValues() const;
    };

    // Contains the NULL time. Use Time::Now() to get the current time.
    Time()
        : TimeBase(0)
    {
    }

    // Returns the time for epoch in Unix-like system (Jan 1, 1970).
    static Time UnixEpoch();

    // Returns the current time. Watch out, the system might adjust its clock
    // in which case time will actually go backwards. We don't guarantee that
    // times are increasing, or that two calls to Now() won't be the same.
    static Time Now();

    // Returns the current time. Same as Now() except that this function always
    // uses system time so that there are no discrepancies between the returned
    // time and system time even on virtual environments including our test bot.
    // For timing sensitive unittests, this function should be used.
    static Time NowFromSystemTime();

    // Converts to/from time_t in UTC and a Time class.
    // TODO(brettw) this should be removed once everybody starts using the |Time|
    // class.
    static Time FromTimeT(time_t tt);
    time_t ToTimeT() const;

    // Converts time to/from a double which is the number of seconds since epoch
    // (Jan 1, 1970).  Webkit uses this format to represent time.
    // Because WebKit initializes double time value to 0 to indicate "not
    // initialized", we map it to empty Time object that also means "not
    // initialized".
    static Time FromDoubleT(double dt);
    double ToDoubleT() const;

#if defined(OS_POSIX)
    // Converts the timespec structure to time. MacOS X 10.8.3 (and tentatively,
    // earlier versions) will have the |ts|'s tv_nsec component zeroed out,
    // having a 1 second resolution, which agrees with
    // https://developer.apple.com/legacy/library/#technotes/tn/tn1150.html#HFSPlusDates.
    static Time FromTimeSpec(const timespec& ts);
#endif

    // Converts to/from the Javascript convention for times, a number of
    // milliseconds since the epoch:
    // https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Date/getTime.
    static Time FromJsTime(double ms_since_epoch);
    double ToJsTime() const;

    // Converts to Java convention for times, a number of
    // milliseconds since the epoch.
    int64_t ToJavaTime() const;

#if defined(OS_POSIX)
    static Time FromTimeVal(struct timeval t);
    struct timeval ToTimeVal() const;
#endif

#if defined(OS_MACOSX)
    static Time FromCFAbsoluteTime(CFAbsoluteTime t);
    CFAbsoluteTime ToCFAbsoluteTime() const;
#endif

#if defined(OS_WIN)
    static Time FromFileTime(FILETIME ft);
    FILETIME ToFileTime() const;

    // The minimum time of a low resolution timer.  This is basically a windows
    // constant of ~15.6ms.  While it does vary on some older OS versions, we'll
    // treat it as static across all windows versions.
    static const int kMinLowResolutionThresholdMs = 16;

    // Enable or disable Windows high resolution timer.
    static void EnableHighResolutionTimer(bool enable);

    // Activates or deactivates the high resolution timer based on the |activate|
    // flag.  If the HighResolutionTimer is not Enabled (see
    // EnableHighResolutionTimer), this function will return false.  Otherwise
    // returns true.  Each successful activate call must be paired with a
    // subsequent deactivate call.
    // All callers to activate the high resolution timer must eventually call
    // this function to deactivate the high resolution timer.
    static bool ActivateHighResolutionTimer(bool activate);

    // Returns true if the high resolution timer is both enabled and activated.
    // This is provided for testing only, and is not tracked in a thread-safe
    // way.
    static bool IsHighResolutionTimerInUse();
#endif

    // Converts an exploded structure representing either the local time or UTC
    // into a Time class.
    // TODO(maksims): Get rid of these in favor of the methods below when
    // all the callers stop using these ones.
    static Time FromUTCExploded(const Exploded& exploded)
    {
        base::Time time;
        ignore_result(FromUTCExploded(exploded, &time));
        return time;
    }
    static Time FromLocalExploded(const Exploded& exploded)
    {
        base::Time time;
        ignore_result(FromLocalExploded(exploded, &time));
        return time;
    }

    // Converts an exploded structure representing either the local time or UTC
    // into a Time class. Returns false on a failure when, for example, a day of
    // month is set to 31 on a 28-30 day month.
    static bool FromUTCExploded(const Exploded& exploded,
        Time* time) WARN_UNUSED_RESULT
    {
        return FromExploded(false, exploded, time);
    }
    static bool FromLocalExploded(const Exploded& exploded,
        Time* time) WARN_UNUSED_RESULT
    {
        return FromExploded(true, exploded, time);
    }

    // Converts a string representation of time to a Time object.
    // An example of a time string which is converted is as below:-
    // "Tue, 15 Nov 1994 12:45:26 GMT". If the timezone is not specified
    // in the input string, FromString assumes local time and FromUTCString
    // assumes UTC. A timezone that cannot be parsed (e.g. "UTC" which is not
    // specified in RFC822) is treated as if the timezone is not specified.
    // TODO(iyengar) Move the FromString/FromTimeT/ToTimeT/FromFileTime to
    // a new time converter class.
    static bool FromString(const char* time_string, Time* parsed_time)
    {
        return FromStringInternal(time_string, true, parsed_time);
    }
    static bool FromUTCString(const char* time_string, Time* parsed_time)
    {
        return FromStringInternal(time_string, false, parsed_time);
    }

    // Fills the given exploded structure with either the local time or UTC from
    // this time structure (containing UTC).
    void UTCExplode(Exploded* exploded) const
    {
        return Explode(false, exploded);
    }
    void LocalExplode(Exploded* exploded) const
    {
        return Explode(true, exploded);
    }

    // Rounds this time down to the nearest day in local time. It will represent
    // midnight on that day.
    Time LocalMidnight() const;

private:
    friend class time_internal::TimeBase<Time>;

    explicit Time(int64_t us)
        : TimeBase(us)
    {
    }

    // Explodes the given time to either local time |is_local = true| or UTC
    // |is_local = false|.
    void Explode(bool is_local, Exploded* exploded) const;

    // Unexplodes a given time assuming the source is either local time
    // |is_local = true| or UTC |is_local = false|. Function returns false on
    // failure and sets |time| to Time(0). Otherwise returns true and sets |time|
    // to non-exploded time.
    static bool FromExploded(bool is_local,
        const Exploded& exploded,
        Time* time) WARN_UNUSED_RESULT;

    // Converts a string representation of time to a Time object.
    // An example of a time string which is converted is as below:-
    // "Tue, 15 Nov 1994 12:45:26 GMT". If the timezone is not specified
    // in the input string, local time |is_local = true| or
    // UTC |is_local = false| is assumed. A timezone that cannot be parsed
    // (e.g. "UTC" which is not specified in RFC822) is treated as if the
    // timezone is not specified.
    static bool FromStringInternal(const char* time_string,
        bool is_local,
        Time* parsed_time);

    // Comparison does not consider |day_of_week| when doing the operation.
    static bool ExplodedMostlyEquals(const Exploded& lhs, const Exploded& rhs);
};

// static
/*constexpr*/ inline TimeDelta TimeDelta::FromDays(int days)
{
    return days == std::numeric_limits<int>::max()
        ? Max()
        : TimeDelta(days * Time::kMicrosecondsPerDay);
}

// static
/*constexpr*/ inline TimeDelta TimeDelta::FromHours(int hours)
{
    return hours == std::numeric_limits<int>::max()
        ? Max()
        : TimeDelta(hours * Time::kMicrosecondsPerHour);
}

// static
/*constexpr*/ inline TimeDelta TimeDelta::FromMinutes(int minutes)
{
    return minutes == std::numeric_limits<int>::max()
        ? Max()
        : TimeDelta(minutes * Time::kMicrosecondsPerMinute);
}

// static
/*constexpr*/ inline TimeDelta TimeDelta::FromSeconds(int64_t secs)
{
    return FromProduct(secs, Time::kMicrosecondsPerSecond);
}

// static
/*constexpr*/ inline TimeDelta TimeDelta::FromMilliseconds(int64_t ms)
{
    return FromProduct(ms, Time::kMicrosecondsPerMillisecond);
}

// static
/*constexpr*/ inline TimeDelta TimeDelta::FromSecondsD(double secs)
{
    return FromDouble(secs * Time::kMicrosecondsPerSecond);
}

// static
/*constexpr*/ inline TimeDelta TimeDelta::FromMillisecondsD(double ms)
{
    return FromDouble(ms * Time::kMicrosecondsPerMillisecond);
}

// static
/*constexpr*/ inline TimeDelta TimeDelta::FromMicroseconds(int64_t us)
{
    return TimeDelta(us);
}

// static
/*constexpr*/ inline TimeDelta TimeDelta::FromDouble(double value)
{
    // TODO(crbug.com/612601): Use saturated_cast<int64_t>(value) once we sort out
    // the Min() behavior.
    return value > std::numeric_limits<int64_t>::max()
        ? Max()
        : value < -std::numeric_limits<int64_t>::max()
            ? -Max()
            : TimeDelta(static_cast<int64_t>(value));
}

// static
/*constexpr*/ inline TimeDelta TimeDelta::FromProduct(int64_t value,
    int64_t positive_value)
{
    return (
#if !defined(_PREFAST_) || !defined(OS_WIN)
        // Avoid internal compiler errors in /analyze builds with VS 2015
        // update 3.
        // https://connect.microsoft.com/VisualStudio/feedback/details/2870865
        DCHECK(positive_value > 0),
#endif
        value > std::numeric_limits<int64_t>::max() / positive_value
            ? Max()
            : value < -std::numeric_limits<int64_t>::max() / positive_value
                ? -Max()
                : TimeDelta(value * positive_value));
}

// For logging use only.
BASE_EXPORT std::ostream& operator<<(std::ostream& os, Time time);

// TimeTicks ------------------------------------------------------------------

// Represents monotonically non-decreasing clock time.
class BASE_EXPORT TimeTicks : public time_internal::TimeBase<TimeTicks> {
public:
    // The underlying clock used to generate new TimeTicks.
    enum class Clock {
        LINUX_CLOCK_MONOTONIC,
        IOS_CF_ABSOLUTE_TIME_MINUS_KERN_BOOTTIME,
        MAC_MACH_ABSOLUTE_TIME,
        WIN_QPC,
        WIN_ROLLOVER_PROTECTED_TIME_GET_TIME
    };

    TimeTicks()
        : TimeBase(0)
    {
    }

    // Platform-dependent tick count representing "right now." When
    // IsHighResolution() returns false, the resolution of the clock could be
    // as coarse as ~15.6ms. Otherwise, the resolution should be no worse than one
    // microsecond.
    static TimeTicks Now();

    // Returns true if the high resolution clock is working on this system and
    // Now() will return high resolution values. Note that, on systems where the
    // high resolution clock works but is deemed inefficient, the low resolution
    // clock will be used instead.
    static bool IsHighResolution();

#if defined(OS_WIN)
    // Translates an absolute QPC timestamp into a TimeTicks value. The returned
    // value has the same origin as Now(). Do NOT attempt to use this if
    // IsHighResolution() returns false.
    static TimeTicks FromQPCValue(LONGLONG qpc_value);
#endif

    // Get an estimate of the TimeTick value at the time of the UnixEpoch. Because
    // Time and TimeTicks respond differently to user-set time and NTP
    // adjustments, this number is only an estimate. Nevertheless, this can be
    // useful when you need to relate the value of TimeTicks to a real time and
    // date. Note: Upon first invocation, this function takes a snapshot of the
    // realtime clock to establish a reference point.  This function will return
    // the same value for the duration of the application, but will be different
    // in future application runs.
    static TimeTicks UnixEpoch();

    // Returns |this| snapped to the next tick, given a |tick_phase| and
    // repeating |tick_interval| in both directions. |this| may be before,
    // after, or equal to the |tick_phase|.
    TimeTicks SnappedToNextTick(TimeTicks tick_phase,
        TimeDelta tick_interval) const;

    // Returns an enum indicating the underlying clock being used to generate
    // TimeTicks timestamps. This function should only be used for debugging and
    // logging purposes.
    static Clock GetClock();

#if defined(OS_WIN)
protected:
    typedef DWORD (*TickFunctionType)(void);
    static TickFunctionType SetMockTickFunction(TickFunctionType ticker);
#endif

private:
    friend class time_internal::TimeBase<TimeTicks>;

    // Please use Now() to create a new object. This is for internal use
    // and testing.
    explicit TimeTicks(int64_t us)
        : TimeBase(us)
    {
    }
};

// For logging use only.
BASE_EXPORT std::ostream& operator<<(std::ostream& os, TimeTicks time_ticks);

// ThreadTicks ----------------------------------------------------------------

// Represents a clock, specific to a particular thread, than runs only while the
// thread is running.
class BASE_EXPORT ThreadTicks : public time_internal::TimeBase<ThreadTicks> {
public:
    ThreadTicks()
        : TimeBase(0)
    {
    }

    // Returns true if ThreadTicks::Now() is supported on this system.
    static bool IsSupported()
    {
#if (defined(_POSIX_THREAD_CPUTIME) && (_POSIX_THREAD_CPUTIME >= 0)) || (defined(OS_MACOSX) && !defined(OS_IOS)) || defined(OS_ANDROID)
        return true;
#elif defined(OS_WIN)
        return IsSupportedWin();
#else
        return false;
#endif
    }

    // Waits until the initialization is completed. Needs to be guarded with a
    // call to IsSupported().
    static void WaitUntilInitialized()
    {
#if defined(OS_WIN)
        WaitUntilInitializedWin();
#endif
    }

    // Returns thread-specific CPU-time on systems that support this feature.
    // Needs to be guarded with a call to IsSupported(). Use this timer
    // to (approximately) measure how much time the calling thread spent doing
    // actual work vs. being de-scheduled. May return bogus results if the thread
    // migrates to another CPU between two calls. Returns an empty ThreadTicks
    // object until the initialization is completed. If a clock reading is
    // absolutely needed, call WaitUntilInitialized() before this method.
    static ThreadTicks Now();

#if defined(OS_WIN)
    // Similar to Now() above except this returns thread-specific CPU time for an
    // arbitrary thread. All comments for Now() method above apply apply to this
    // method as well.
    static ThreadTicks GetForThread(const PlatformThreadHandle& thread_handle);
#endif

private:
    friend class time_internal::TimeBase<ThreadTicks>;

    // Please use Now() or GetForThread() to create a new object. This is for
    // internal use and testing.
    explicit ThreadTicks(int64_t us)
        : TimeBase(us)
    {
    }

#if defined(OS_WIN)
    FRIEND_TEST_ALL_PREFIXES(TimeTicks, TSCTicksPerSecond);

    // Returns the frequency of the TSC in ticks per second, or 0 if it hasn't
    // been measured yet. Needs to be guarded with a call to IsSupported().
    // This method is declared here rather than in the anonymous namespace to
    // allow testing.
    static double TSCTicksPerSecond();

    static bool IsSupportedWin();
    static void WaitUntilInitializedWin();
#endif
};

// For logging use only.
BASE_EXPORT std::ostream& operator<<(std::ostream& os, ThreadTicks time_ticks);

} // namespace base

#endif // BASE_TIME_TIME_H_
